The transmission of digital data at high speeds, e.g., 9600 bits per second and higher, over band-limited transmission channels, such as telephone voice channels, requires precision control over impairments resulting from linear distortion. The instrumentality widely applied to compensate for linear impairments like amplitude and delay distortion, which give rise to intersymbol interference, is the transversal equalizer which generates a linear combination of selectively weighted consecutive samples of received data signals. The selective weights exist as a plurality of tap-gain values applied to delayed replicas of synchronous samples of received data signals spread over a tapped delay line or shift register. Typically, the tap-gain values are determined according to a performance criterion which minimizes the mean-square error difference between the actual equalizer output and discrete permissible outputs. Such error difference is then correlated with each of the tap outputs to adjust individual tap-gain values in accordance with the gradient of the error with respect to the existing tap-gain value.
The process of obtaining optimum tap-gain values adaptive to training sequences or to raw data is iterative. The time required for initial convergence to optimum values is called settling time, and for severely distorted transmission channels, can be projected beyond practical limits for many data applications. In polling situations, for example, messages are frequent but short, with the possible result that settling time can exceed message time.
Prior proposals for the reduction of settling time have involved the orthogonalization of the transmission channel correlation matrix, a matrix formed from all the possible cross-products of the signal samples incident on the delay-line structure during each signaling interval. Orthogonalization accelerates convergence, and has been achieved in a relatively straightforward manner when the channel characteristics are known in advance. Attempts to perform an orthogonalization adaptively have resulted in complex equalizer structures which create minimal reduction in overall settling time when orthogonalization time itself is included.
It is an object of this invention to provide rapid adaptive self-orthogonalization of data signal samples stored in a tapped delay line in order to reduce settling time in transversal equalizers.
It is another object of this invention to accelerate the initial convergence of tap gain coefficients in transversal equalizers for digital data transmission systems to optimum values.
It is a further object of this invention to implement a self-orthogonalizing gradient adjustment algorithm for linear tapped delay line transversal equalizers in digital data transmission systems to reduce settling time for equalizer tap weights which minimize mean-square error.